The long range goal of the proposed project is to provide a better understanding of how the efferent vestibular system is involved in the overall balance function of the inner ear. Vestibular afferent neurons transmit very accurate head-movement information from the inner ear to the brain. Efferent neurons allow the brain to modify information coming from the inner ear. Despite numerous studies on the efferent vestibular system, the influence of this system on incoming afferent information remains obscure. The efferent system is known to be "turned on" in anticipation of active movements. Therefore, it seems likely that it functions in some way to optimize our sensing of a motion, which in turn will optimize balance and coordination. Previous studies have indicated that the efferent system may help prevent the vestibular afferent neurons from being over-driven by strong movement stimuli. Part of this effect involves an apparent reduction in afferent sensitivity. However, previous investigators have not quantitatively addressed afferent sensitivity with respect to the background "noise" present in the neuron's discharge activity. Because the efferent system can reduce this temporal noise level (coefficient of variation in interspike intervals), it is quite possible that the overall effect of the efferent system on many afferent neurons is to increase their sensitivity. Aside from sensitivity, other aspects of vestibular afferent response dynamics may be influenced by the efferent system. The dynamics of an afferent neuron's response to a rotational motion reflect how much acceleration versus velocity information is carried by that particular neuron. During natural, quick head movements, the efferent system may alter, in some cases, afferent response dynamics in favor of acceleration or may optimize linearity of the velocity response in other cases. The proposed research will use single-unit, microelectrode recording techniques in conjunction with efferent stimulation and pharmacological manipulations. Evoked responses will be analyzed both in the time and frequency domains. A thorough analysis will be made of spike train noise and afferent response dynamics in the presence and absence of efferent stimulation. These investigations should lead to a more quantitative understanding of efferent vestibular effects on afferent sensitivity and responses to rotational movements than currently exists. This in turn will likely provide new insights into normal vestibular function as well as the role of the efferent vestibular system in human balance disorders and recovery from vestibular injury.